557
ACUS01 KWNS 051251
SWODY1
SPC AC 051250

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0750 AM CDT Sun May 05 2024

Valid 051300Z - 061200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
SOUTH TEXAS...

...SUMMARY...
Scattered strong to severe thunderstorms are possible across parts
of South Texas today.  Isolated locally severe thunderstorms are
also possible across the Mid-South and upper Ohio Valley.

...Synopsis...
A substantial synoptic-scale trough has moved inland over the
Pacific Coast States and NV, anchored by a cyclone now centered near
the NV/OR border.  The 500-mb low should move erratically eastward
through the period, reaching southern WY by 12Z tomorrow, with a
strong shortwave trough southward over western CO and NM.
Associated height falls will shift eastward through the
Intermountain West, Rockies, and eventually High Plains.  As that
occurs, a convectively reinforced, southern-stream perturbation --
apparent in moisture-channel imagery from western OK to central/
southeast TX -- will move eastward to the lower Mississippi Valley.
An embedded MCV, initially evident over southern OK, should move
northeastward toward the Ozarks through 00Z.  Another MCV may be
located over northeast TX, and should move over AR today.

At the surface, 11Z analysis showed a weak low over northwestern OH,
near a frontal triple point, with warm front arching across eastern
OH, western WV and eastern KY.  This front represents the western
rim of a cold-damming airmass over the central Appalachians, and
will be slow to erode from the west as the cold front catches up
today.  The cold front extended from the low across portions of
northern/western KY to northern AR, to a frontal-wave low over
northeastern OK, related to the northern MCV.  The front then
extended across eastern OK, and north-central/southwest TX.  The
trailing part of the cold front is expected to lose definition
amidst:
1.  A vast field of convective outflow air already in place across
much of central/north/east TX, OK and AR, as well as
2.  Intensifying low-level warm advection on both sides late in the
period, contributing to frontolysis.

...South TX to Mid-South/lower Mississippi Valley...
A dissipating convective complex is moving eastward across portions
of LA, followed by a still-robust but mostly non-severe line of
thunderstorms over southeast TX.  The trailing line may produce
isolated gusts near severe limits for a few more hours, but should
move into an airmass modified by the leading activity.  Meanwhile,
the boundary layer farther north -- ahead of a belt of precip and
embedded thunderstorms now over eastern OK and the Arklatex --
should destabilize diabatically through the afternoon.  That, along
with favorable low-level moisture (surface dewpoints generally in
the mid/upper 60s F), should offset modest mid/upper-level lapse
rates enough to support peak MLCAPE in the 1000-1500 J/kg range over
much of the outlook area.  Scattered thunderstorms are expected --
perhaps in the form of intensification along the northeastward-
shifted leading edge of the current convection.  With modest
low-level and deep shear, severe potential should remain marginal
and isolated, in the form of gusts and hail.  Mesobeta-scale
mass-response effects ahead of the MCVs might help organization over
parts of the Mid-South region later today.

Farther southwest, the aggregate outflow boundary from the morning
complexes as analyzed from near HOU-COT and into northern Coahuila.
This boundary should stall, then drift northward over south-central
TX and the adjoining Rio Grande Valley today.  The boundary should
focus additional thunderstorm development from midday through the
afternoon, some of which may become severe multicells or supercells
with large hail, damaging gusts and at least marginal tornado
potential.  Though flow beneath the 700-mb level should be modest
(generally 5-10 kt), two factors will favor severe potential:
1.  Backed flow near the boundary, with maximized low-level
vorticity, augmentation of weak ambient shear, and favorable
storm-relative low-level winds on the mesoscale.
2.  Favorable buoyancy, with MLCAPE commonly 2000-3000 J/kg along
and south of the boundary, will be fostered by strong diurnal
heating, rich low-level moisture (70s F surface dewpoints) and weak
CINH.

In the absence of notable mid/upper support behind the shortwave
trough, convection may diminish rapidly through the evening, as the
boundary layer stabilizes from both nocturnal cooling and spreading
outflow air.

...Upper Ohio Valley vicinity...
Episodes of widely scattered to scattered thunderstorms are expected
to move roughly eastward across this outlook area today, with
isolated damaging (but mainly subsevere) gusts possible.
Activity will occur as weak MLCINH is breached by diabatic heating
and frontal lift.  In the pocket of favorable boundary-layer air
between the warm and cold fronts, 60s F surface dewpoints should
contribute to MLCAPE in the 1000-1500 J/kg range.  Glancing
influences of a northern-stream trough -- passing north of the area
from the Upper Great Lakes across northeastern ON and adjoining QC
-- will include somewhat tightened height gradients and enhanced
mid/upper winds.  However, low-level flow will remain weak and
veered to southwesterly, with weak vertical shear in the lowest 500
mb.  Mode therefore should be multicellular, with convection
weakening considerably as low-level instability decreases near and
after sunset.

..Edwards/Broyles.. 05/05/2024

$$